From the operation of internal combustion engines with a plurality of piston-cylinder units (in short: cylinders), it is known that the ignition pressures or the mean values of the ignition pressures of the individual piston-cylinder units differ from one another by different influencing factors. Influencing factors which lead to such a variation of the ignition pressures are e.g. oil and propellant gas deposits in the combustion chamber, different flow conditions of the cylinder head spin, spark plug contamination, different intake temperatures in individual cylinders, manufacturing tolerances which influence the compression ratio, valve clearance and valve clearance changes, differences in the air ratio in individual cylinders, pressure oscillations in the intake system, crankshaft vibrations, camshaft vibrations, etc.
This spreading of the ignition pressures (expressed as peak firing pressure, PFP) is undesirable, since the mobile operating point of the engine shifts strongly in the engine characteristic map (e.g. misfire limit, knock limit, limit due to the limited maximum exhaust gas turbocharger speed and maximum turbine intake temperature). At the same time, it is desirable to operate the internal combustion engine as close as possible to the knocking limit of the piston-cylinder units, since this is the operating point with the highest efficiency or the operating point is very close to the misfire limit to reduce nitrogen oxide emissions. Cylinders with a high peak firing pressure already run closer to the knocking limit than cylinders with lower peak firing pressure, meaning that spreading of the ignition pressures is also undesirable with regard to the achievable efficiency.
Equalization of the piston-cylinder units with regard to their peak firing pressure is usually performed in the prior art via cylinder pressure sensors or alternative sensors, which can supply a signal which is characteristic of the peak firing pressure. Examples are sensors for measuring the combustion chamber temperature or ion current sensors.
DE 10 2004 041230 A1, for example, discloses a method for cylinder equalization in which maximum cylinder pressures are determined in an individual cylinder by means of an ion current measurement and mean values of the maximum pressures are formed, whereby at least one cylinder is identified, as a function of a comparison between cylinder groups, whose operating behavior is to be influenced.
It is known from DE 19605803 A1 that an ion current signal can be used to detect the knocking of the internal combustion engine and to display a corresponding knock control by means of control of the ignition time.
U.S. Pat. No. 7,383,816 B2 discloses a method for operating an internal combustion engine in which an advance of an ignition time of a first subset of piston-cylinder units is performed from an operating ignition time until a knocking event is detected, while simultaneously operating the remaining piston-cylinder units at the operating ignition time. A first knock limit of the first subset of piston-cylinder units is thus determined with respect to a difference between the operating ignition time and the ignition time at the time of the knock event. By advancing the ignition sequence in a second subset of piston-cylinder units from an operating ignition time while simultaneously operating the remaining piston-cylinder units at the operating ignition time, a second knock limit of the second subset of piston-cylinder units is determined in relation to a difference between the operating ignition time and the ignition time at the moment of the knock event. This serves the purpose of determining a characteristic of a propellant which is supplied to the piston-cylinder units with respect to the first and second knock limits. By means of this method, the knock resistance of a fuel supplied to the internal combustion engine can be determined. The ignition time of the internal combustion engine can then be adapted to the knock resistance of the fuel.